Organic isocyanates are generally prepared on a large commercial scale by reacting the corresponding amines with phosgene. Because of the toxicity of phosgene, many attempts have been made to find a suitable method of synthesizing organic isocyanates on a large commercial scale without the use of phosgene. One such method consists of reacting organic nitro compounds with carbon monoxide and organic hydroxyl compounds to form the corresponding urethanes, followed by decomposition of the urethanes into isocyanates and hydroxyl compounds. Modification of the urethane obtained as an intermediate product before decomposition is also possible. Thus, for example, phenyl urethane which is obtainable from nitro benzene, carbon monoxide and ethanol could first be reacted with formaldehyde to form the bis-urethane of 4,4'-diisocyanatodiphenyl methane which could then be converted into 4,4'-diisocyanatodiphenyl methane by elimination of the ethanol.
The decomposition of urethanes into the corresponding isocyanates and hydroxyl compounds has been described, for example, in German Offenlegungsschrift No. 2,421,503 and the prior publications cited therein.
The reactions described in the patent literature for the preparation of urethanes include the reaction of nitro compounds with carbon monoxide and alcohols in the presence of selenium or selenium compounds as described in German Offenlegungsschriften Nos. 2,343,826; 2,614,101 and 2,623,694 or of noble metals, in particular palladium, in the presence of Lewis acids as described in German Offenlegungsschriften Nos. 1,568,044 and 2,603,574. For the preparation of a mono nitro compound this reaction proceeds in accordance with the following stoichiometric equation: EQU R--NO.sub.2 +3CO+R'OH.fwdarw.RNHCO.sub.2 R'+2CO.sub.2.
The general reaction equation is as follows: EQU R(NO.sub.2).sub.x +3xCO+xR'OH.fwdarw.R(NHCO.sub.2 R').sub.x +2xCO.sub.2.
This means that for every mol of urethane group produced, 3 mol of carbon monoxide are used up and 2 mol of carbon dioxide are formed. Only one-third of the carbon monoxide put into the process is thus used for the formation of the urethane group while two-thirds are converted into the industrially valueless inert carbon dioxide. Because of the large quantity of heat produced in the exothermic formation of carbon dioxide, expensive apparatus for removal of this heat is required in the known industrial process of synthesizing urethane based on the use of nitro compounds, carbon monoxide and alcohol.
Chemistry Letters (Chemical Society of Japan), 1972, pages 373-374, describes reacting primary amines with stoichiometric quantities of selenium, a tertiary amine, carbon monoxide and alcohol to produce a complex salt from which the corresponding urethane is obtained by reaction with molecular oxygen, the selenium and tertiary amine both being recovered. This can be shown as follows: ##STR1##
The overall reaction results in the formation of urethane in accordance with the following equation: EQU RNH.sub.2 +CO+R'OH+1/20.sub.2 .fwdarw.RNHCO.sub.2 R'+H.sub.2 O.
This overall reaction requires only 1 mol of carbon monoxide for each urethane group produced. The exothermic heat of reaction is, therefore, less since only 1 mol of water is formed instead of the 2 mol of carbon dioxide produced when nitro compounds are used. This oxycarbonylation "catalyst" with stoichiometric quantities of selenium is not, however, suitable for the synthesis of urethane on a commercial scale. This is because the method requires the use of unacceptably large quantities of selenium which is toxic, expensive and difficult to recover quantitatively from the reaction mixture. Moreover, the oxycarbonylation using selenium must be carried out in two stages, which adds to the difficulty of using this process on a large scale. Additionally, the yields obtained from this method are unsatisfactorily low.